Up to now, for example, as liquid level detecting means for finding a liquid level in a boiler body, a feed water tank, or the like, or as electric conductivity detecting means for finding an electric conductivity of a boiler water or the like, an electrode holder is mounted in an insulation state to a metal container for detection communicating with inside the boiler body or inside the feed water tank, and one power supply is connected to an electrode of the electrode holder while the other power supply is connected to the metal container to supply power application, for detecting a state of a liquid, such as a liquid level, an electric conductivity, or the like, based on a state of conduction between an electrode of the electrode holder and the metal container.
As the electrode holder to be mounted to the metal container, there is known one including an electrode shaft having one end as a terminal portion and another end as a detection electrode portion, and a container mounting portion having a through-hole and made of metal, for holding the electrode shaft in the through-hole through an intermediation of an insulator formed of a synthetic resin, and having a structure in which the electrode shaft is mounted to the metal container by the container mounting portion in a state where the electrode shaft passes through a hole portion of the metal container while being electrically insulated from the metal container by the insulator (see JP-A 2000-46628).
According to the electrode holder, for example, when an inside is in a high-temperature state like in a case of the boiler body, an inside of the metal container communicating therewith is also in the high-temperature state. Therefore, there is a fear that the insulator of the electrode holder mounted to the metal container and exposed to the inside of the metal container cannot resist the high temperature, to thereby be destroyed.
When the insulator is destroyed as described above, there is a fear of the electrode shaft held by the container mounting portion coming out of the container mounting portion to fall off into the metal container. In particular, when a negative pressure exists in the metal container, there is a fear that the electrode shaft falls off to be drawn into the metal container, thereby damaging the metal container. Further, when a high pressure exists inside like in the case of the boiler body, a high pressure also exists in the metal container, so there is a fear that due to the destruction of the insulator, the electrode shaft held by the container mounting portion comes out of the container mounting portion to burst out of the metal container, and a liquid in the metal container is issued from a gap generated between the electrode shaft and the container mounting portion.
Further, the electrode shaft and the container mounting portion are each formed of metal while the insulator is formed of a synthetic resin, so the electrode shaft and the insulator, and the container mounting portion and the insulator easily slide with each other, respectively. Due to a difference in coefficient of linear expansion between the electrode shaft or the container mounting portion formed of metal and the insulator formed of a synthetic resin, and a mold shrinkage factor of the synthetic resin, or when a torsional torque is applied to the electrode shaft or the container mounting portion, there is a fear that abrasion is caused between the electrode shaft and the insulator or between the container mounting portion and the insulator, thereby causing a liquid in the metal container to leak from a gap generated by the abrasion.